Welsh police arrested four people and are combing the grounds of a stable outside the capital city, Cardiff, on Tuesday, hunting for a body in a long-running slavery investigation.

Gwent Police said in a statement that more than 100 officers are involved in searches at the yard, as well as an apartment in Cardiff and a house in the small Welsh village of Penhow. Those arrested are between the ages of 36 and 66.

The force said it was acting on suspicion that people were being kept in poor conditions at the site “and forced to work for no pay.” Police said that an unnamed man – believed to be a Polish national – was recovered from the site on Monday and is currently being assessed by police and British Red Cross personnel.

Police added that horses found at the stable are being examined by animal welfare experts.

There were few other details about the nature of the investigation or why police believed a body might be hidden somewhere in the area. Pictures published by news websites WalesOnline showed black-clad officers picking through the underbrush with rods, and an excavator being deployed to the site.

Police said that the investigation was sparked by the discovery of a man who was found living “in very poor conditions” on the site earlier this year.

WalesOnline identified the man as Darrell Simester, who disappeared following an argument with friends roughly 13 years ago. The paper, which described Simester as vulnerable and timid, said his family received a call from two men several days after he went missing saying that Simester was “working on the roads” with them.